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https://issues.apache.org/jira/browse/SPARK-31430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17211286#comment-17211286
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Sean R. Owen commented on SPARK-31430:
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Sounds good, I usually mark as a Duplicate.
> Bug in the approximate quantile computation.
> --------------------------------------------
>
> Key: SPARK-31430
> URL: https://issues.apache.org/jira/browse/SPARK-31430
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 3.0.0
> Reporter: Siddartha Naidu
> Priority: Major
> Attachments: approx_quantile_data.csv
>
>
> I am seeing a bug where passing lower relative error to the
> {{approxQuantile}} function is leading to incorrect result in the presence of
> partitions. Setting a relative error 1e-6 causes it to compute equal values
> for 0.9 and 1.0 quantiles. Coalescing it back to 1 partition gives correct
> results. This issue was not present in spark version 2.4.5, we noticed it
> when testing 3.0.0-preview.
> {{>>> df = spark.read.csv('file:///tmp/approx_quantile_data.csv',
> header=True,
> schema=T.StructType([T.StructField('Store',T.StringType(),True),T.StructField('seconds',T.LongType(),True)]))}}
> {{>>> df = df.repartition(200, 'Store').localCheckpoint()}}
> {{>>> df.approxQuantile('seconds', [0.8, 0.9, 1.0], 0.0001)}}
> {{[1422576000.0, 1430352000.0, 1438300800.0]}}
> {{>>> df.approxQuantile('seconds', [0.8, 0.9, 1.0], 0.00001)}}
> {{[1422576000.0, 1430524800.0, 1438300800.0]}}
> {color:#de350b}{{>>> df.approxQuantile('seconds', [0.8, 0.9, 1.0],
> 0.000001)}}{color}
> {color:#de350b}{{[1422576000.0, 1438300800.0, 1438300800.0]}}{color}
> {{>>> df.coalesce(1).approxQuantile('seconds', [0.8, 0.9, 1.0], 0.000001)}}
> {{[1422576000.0, 1430524800.0, 1438300800.0]}}
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